1. Field of the Invention
The present invention relates to a method for increasing dimensional stability, densification, void elimination and internal bonding between compactable particulates, preferably conductive and refractory constituents, within contact members used in switches, circuit breakers, and a wide variety of other applications.
2. Description of the Prior Art
Electrical contacts, used in circuit breakers and other electrical devices, contain constituents with capabilities to efficiently conduct high flux energy from arcing surfaces, while at the same time resist erosion by melting and/or evaporation at the arc attachment points. During interruption, where currents may be as high as 200,000 amperes, local current densities can approach 10.sup.5 amps/cm.sup.2 at anode surfaces and up to 10.sup.8 amps/cm.sup.2 at cathode surfaces on contacts. Transient heat flux can range up to 10.sup.6 KW/cm.sup.2 at arc roots, further emphasizing the demand for contact materials of the highest thermal and electrical conductivity, and either silver or copper is generally selected. Silver is typically selected in air break applications, where post-arc surface oxidation would otherwise entail high electrical resistance on contact closure. Copper is generally preferred where other interrupting mediums (oil, vacuum or sulfur hexafluoride) preclude surface oxidation.
Despite the selection of contact metals having the highest conductivity, transient heat flux levels such as that previously mentioned, result in local surface temperatures far exceeding the contact melting point (962.degree. C. and 1083.degree. C. for silver and copper, respectively), and rapid erosion would result if either would be used exclusively. For this reason, a second material, generally graphite, or a high melting point refractory metal such as tungsten or molybdenum, or a refractory carbide, nitride and/or boride, is used in combination with the highly conductive metal to retard massive melting.
Conventional contact production processes generally involve blending powdered mixtures of high conductivity and high melting point materials, and pressing them into contacts, which are then thermally sintered in reducing or inert gas atmospheres. After sintering, the contacts are then infiltrated with conductive metal, which involves placing a conductive metal "slug" onto each contact and heating it in a reducing (or inert) gas atmosphere, this time above the conductor's melting point. The contacts may then be re-pressed, to increase density to levels of 96% to 98% of theoretical and then post-treated for final installation into the switching device.
These approaches have several disadvantages, in that they have limited process versatility, consist of numerous process steps resulting in a high cost operation, and have a limitation in the achievable densities and performance characteristics. U.S. Pat. No. 4,810,289 (N. S. Hoyer et al.) solved many of these problems, by utilizing highly conductive Ag or Cu, in mixture with CdO, W, WC, Co, Cr, Ni, or C, and by providing oxide clean metal surfaces in combination with a controlled temperature, hot isostatic pressing operation. There, the steps included cold, uniaxial pressing; canning the pressed contacts in a container with separating aid powder; evacuating the container; and hot isostatically pressing the contacts.
The Hoyer et al. process provided full density, high strength contacts, with enhanced metal-to-metal bonds. Such contacts had minimal delamination after arcing, with a reduction in arc root erosion rate. However, such contacts suffered from volumetric shrinkage during processing. What is needed is a method to provide dimensionally predictable and reproducible contacts which would shrink, if at all, only in one direction during processing, while still maintaining high strength, resistance to delamination, and enhanced metal-to-metal bonding characteristics. It is a main object of this invention to provide a method of making such superior contacts.